Females suffer more than males from mental illness, most forms of which first become apparent at adolescence. Girls may also experience a more tumultuous adolescence than do boys. In addition, there are recent trends toward increased delinquency on the part of girls and adolescent girls, more than boys, are showing declines in school achievement. There is accumulating evidence that early adolescence is the most critical phase of adolescence, particularly for female development. Although it is frequently assumed that many psychosocial changes experienced by developing females are linked to pubertal changes, there has been relatively little research examining the relationship of biological development to other aspects. This study systematically investigates biopsychosocial development in early adolescent males and females. In addition, the parents of these young people are also studied. The design includes two cohorts of early adolescents, each followed for three years beginning at sixth grade. The sample sizes for the two cohorts are 188 and 168, respectively, with approximately equal numbers of boys and girls. Two control groups, a retest control group and a dropout control group, are also included. The primary assessment is a semi-structured itnterview conducted twice annually. In addition, pubertal development is assessed semi-annually and cognitive development and functioning are assessed annually. The data from the interview are validated by data obtained annually on self-image, psychological characteristics, and gender identity. Parents are interviewed twice during the study. School data are also obtained. This study will provide basic developmental data currently lacking on early adolescents. Specific questions about the development of sex-related differences will be addressed. The results will be significant for our understanding of development and how we might most effectively intervene for the benefit of youth.